A planet is a large spherical body, directly orbiting a star (but not being itself a star). To qualify as a planet, the body must be massive enough to have its spherical (or more accurately ellipsoidal shape) by virtue of its own gravity; a smaller body artificially shaped into a sphere would not be classified as a planet.
Although some planets are habitable by humans and other organisms, not all habitable worlds are planets. By definition, a planet must be spherical in shape, and must be in orbit around a sun. Planets are primarily known in the multiverse of Yacun, in the cosmos of Xi, although it is certainly conceivable that sufficiently similar bodies may exist on some planes of other cosmoi to warrant similar terminology. In any case, worlds which do not satisfy all the criteria of planets but which, like planets, comprise discrete areas which may compose varied terrains, especially if they are habitable, are known as "planetary worlds", or "mounds".
In Yacun, at least, planets are believed to be formed by the accretion of protoplanetary disks, which in turn form from the collapse of a nebula. The center of the disk condenses into a protostar, while particles in the outer regions of the disk collide and combine into numerous larger bodies called planetesimals. The process continues, the planetesimals joining into protoplanets, and finally into planets as they clear their orbits and gravitationally capture remaining smaller bodies as moons. As the protostar ignites into a full-fledged star, the solar wind and other processes clear most of the remaining dust and small particles from the disk.
Once a planet has formed, its further evolution depends on its mass and composition. Initially liquid planets may solidify as they cool down, though their interiors may retain some warmth from their formation, as well as still generating some heat from radioactivity. Their surfaces may be reshaped by meteor impacts and by collision with other bodies—the prevailing theory on the origin of the Earth's Moon is that it started as part of the Earth knocked out by such a collision. If life originates on a planet, that life may further alter the chemical makeup of the planet and its atmosphere through its metabolic processes.
Planets are not generally found by themselves, but as part of a planetary system in orbit around a sun. This planetary system includes not only the planets that formed from the same disk, but also other bodies in orbit around the sun or the planets such as moons, comets, and meteoroids. Because they all formed from the same rotating disk, the planets will generally all orbit the sun in the same direction, though their axial inclinations may vary.
It may be possible for a planet to escape the gravity of its sun and travel through space on its own, conceivably retaining enough warmth from radiation to maintain an atmosphere, and possibly in some cases even keeping its moons. Such extrasystemic bodies are often called rogue planets, though some astronomers dislike this term, preferring to reserve the word "planet" for objects actually in orbit around a star.
Types of planet
Planets vary widely in their sizes and their composition. There are limits to their sizes, of course—too small and they won't be spherical; too large and they'll undergo thermonuclear fusion and become stars. Still, that leaves a wide range open—the largest planet in the Earth's solar system, Jupiter, has more than five thousand times the mass of Mercury, the smallest. Because knowledge of planets outside the Earth's solar system is currently very incomplete, it's hard to know how typical the Earth's system is, and any generalizations from this one system are inherently questionable. However, based on this necessarily limited information, at least three basic types of planets have been identified. Planets like the Earth, made up mostly of various sorts of rock, are known as terrestrials. Planets made mostly of gas are known as gas giants; as the name implies, they tend to be much larger than terrestrials—Jupiter is a gas giant; Mercury a terrestrial. Finally, there are planets similar to gas giants, but not quite as large, containing a significant amount of ice (not necessarily water ice) in their composition, as well as some rock. Planets of this latter class have traditionally been considered a type of gas giant, but are more specifically called ice giants.
There are indications that some extrasolar planets—that is, planets outside the Earth's system—different from any of the better known planets. The most numerous type of extrasolar planet is the pegasid, or Hot Jupiter, a gas giant orbiting relatively close to its sun. Other possible or hypothetical types of extrasolar planet include the chthonian planet, a rocky sphere originating as the core of a gas giant stripped of its outer layers; the super-Earth, rocky planets significantly larger than the Earth (the Earth is the largest terrestrial planet in its system); and the carbon planet, a rocky planet made mostly of carbon compounds (as opposed to the terrestrial worlds of the Earth's system, which all contain much more silicon than carbon).
In some fictional contexts, coded classification systems have been developed for planets. Probably the best known is that of the Star Trek franchise, which has more than twenty classes, labeled with letters of the alphabet, though all but nine of those have appeared only in one reference book, and may not be considered canonical. (Technically, some of the classes are not planets at all; class D comprises some types of planetoid, for instance.) The best known of the Star Trek planetary classes is class M, for human-habitable worlds, which has become so pervasive that it's even been mentioned in one novel set in the universe of the mostly unrelated franchise of Star Wars, though this may have been an oversight on the part of the writer. The Spelljammer setting for Dungeons & Dragons furnishes another example, though one grounded in a very different type of cosmos and including exotic kinds of planets, such as worlds composed entirely of fire—and also encompassing many bodies that, due to their non-spherical shape, would not be considered planets according to the definition here. The Spelljammer system assigns planets and other celestial bodies to types running from A to J. These types, however, refer only to the size of the body in question, and not to any other qualities. (As a matter of fact, where first introduced, these codes are explicitly referred to only as "size classes", though elsewhere the word "type" is used in reference to them.)
The closest thing to any such classification in use on present-day True Earth is the Sudarsky system, which applies only to non-icy gas giants, assigning them to one of five classes based on their main atmospheric constituents, and especially on the makeup of their clouds. Planets dominated by ammonia clouds are in Class I, by water clouds in Class II, by alkali metals and carbon monoxide in Class IV, and by iron and silicates in Class V. Class III planets do not have global cloud cover, appearing as uniform blue spheres, though they may possess scattered clouds of sulfides and chlorides. These classes run in increasing order of temperature; Class I planets would have surface temperatures of less than 150 Kelvins, running up to over 1400 Kelvins for Class V planets. Both gas giants in the Earth's system, Jupiter and Saturn, would be Class I, but possible planets of all five classes have been observed around other stars. In any case, the Sudarsky scale is still considered speculative, and is by no means universally accepted by astronomers. It's not impossible that a standard classification system will be adopted as more extrasolar planets become known; the existing stellar classification system could be cited as a sort of precedent. However, there are more ways of interest in which planets vary than stars, and if such a system ever is adopted it's perhaps likely to comprise more than a single letter.
Many types of planetary world are known which are not classified as planets. Many universes are full of planetary worlds which do not qualify as planets, either because they are not spherical, or because they do not orbit suns, or both. Even in Yacun, however, there are some bodies which come close to being planets but have some vital distinction. The brown dwarf, for instance, may be the same size as a large planet, but is not generally considered a planet because it has sufficient mass for fusion to take place (even if in practice these bodies do not undergo stable hydrogen fusion). Other bodies similar in mass to planets but formed by different methods and not orbitting stars are sometimes known as planemos.
Recently, Terran astronomers tightened the definition of the planet to require a planet to have cleared the neighborhood around its orbit. Bodies which otherwise fit the definition of a planet but lack this criterion are now known as dwarf planets. Most notoriously, this means that Pluto of the Earth's system, long considered a planet of the same status as the other eight, is now "demoted" to dwarf planet status, a fact that has led to much resistance to the redefinition. However, for the sake of consistency, if Pluto were included as a planet, so too would other, more recently discovered dwarf planets such as Eris, a body more massive than Pluto but orbiting further out, as well as the asteroid Ceres, which had been observed long before Pluto but had not been known to be spherical (and to fit the then-definition of a planet) until relatively recently. This planetary proliferation, in fact, had been the main driving force in the redefinition of the word.